1) The Gemara which states that the Halachah follows Raban Shimon ben Gamliel in the Mishnah apart from those three exceptions is in Bava Basra 174a. The Rif there writes that this rule is "Lav Davka" because we do not say that the Halachah follows Raban Shimon ben Gamliel unless there is a reason to say so.

2) It seems that the Rif is telling us that even though the Gemara gives us this rule, there is a much stronger rule: "Yachid v'Rabim Halachah k'Rabim" -- that the Halachah follows the majority, which appears many times in Shas (see, for instance, Berachos 9a). The rule that the Halachah follows Raban Shimon ben Gamliel in the Mishnah cannot override the rule that the Halachah follows the majority, unless there is a good reason.

3) The Nimukei Yosef to Bava Metzia (40b of the pages of the Rif pages, DH Masnisin Shamin) explains this further. He refers to the Mishnah in Bava Metzia 5:5, in which case the Rif and Rambam do not rule like Raban Shimon ben Gamliel. The Nimukei Yosef writes that despite the rule that the Halachah follows Raban Shimon in the Mishnah, nevertheless the Mefarshim in many places cite the Gemara in Kidushin 34a which tells us that we cannot learn from the rules even in a case where the rule says "with the exception of...." Even if a rule tells us what exceptions there are to it, there still might be other exceptions which we have not been informed of.

4) Here is a specific reason for why the Halachah does not follow Raban Shimon ben Gamliel in the Mishnah on Daf 102. See the Ritva, cited by the Shitah Mekubetzes (102b, DH Rav Nachman), who writes that Rav Nachman, who rules that the land remains in the Chazakah of its owner and thus the landlord always receives the full amount, learns that the Tana of the Mishnah who maintains that they split it equally is Sumchus, who always says to split property in doubt (see Gemara 100a). However, the Halachah follows the Rabanan, who say that one leaves the land in the Chazakah of its owner. The Ritva writes that the Halachah follows Rav Nachman because he is the last opinion cited by the Gemara.

5) So according to the Ritva, Raban Shimon agrees with Sumchus, but Sumchus remains the minority opinion and the rule that the Halachah follows the Rabanan (that "ha'Motzi me'Chavero Alav ha'Re'ayah") against Sumchus is stronger than the rule that the Halachah follows Raban Shimon ben Gamliel in the Mishnah.

Kol Tuv,

Dovid Bloom

Now to the question of whether the Halachah follows Raban Shimon ben Gamliel in Mishnah 9:13.

1) Raban Shimon ben Gamliel states there that the Mitzvah to return the collateral taken against the loan applies only for the first 30 days. After this period, the creditor may sell the collateral in Beis Din. The Rosh (9:47) cites Rabeinu Tam who states that wherever Raban Shimon ben Gamliel said something in the Mishnah, the Halachah follows him, as long as the Gemara does not rule according to an opinion which disagrees with Raban Shimon.

We see that Rabeinu Tam rules like Raban Shimon in 9:13.

However, it seems that Rabeinu Tam does not have to rule like Raban Shimon in the Mishnah on 102b because we may say that the fact that the Gemara there cites Rav Nachman, who does not rule like Raban Shimon, means that the Gemara is ruling against Raban Shimon, since Rav Nachman is a late Amora, and the rule is that we generally rule according to the late Amora'im. In addition, there is a rule that the Halachah follows Rav Nachman in monetary matters.

It should be pointed out, though, that Rabeinu Tam's approach does not seem to be the same as the approach of the Rif that I cited above in my first reply. According to Rabeinu Tam, the Halachah follows Raban Shimon unless something is said to the contrary in the Gemara, while according to the Rif, the onus of proof is the opposite -- there has to be a specific reason to rule like Raban Shimon, and in the absence of such a reason we revert to the general rule that the Halachah follows the majority opinion.

2) The Rosh continues and writes that according to the Rif and the Ge'onim, the Halachah does not follow Raban Shimon. This is because the Gemara on 113b states that Raban Shimon ben Gamliel, Rebbi Shimon, Rebbi Yishmael, and Rabbi Akiva all maintain that "Kol Yisrael Bnei Melachim," according to which it follows that one may take away items from the debtor. According to the Rif, we do not rule like these four Tana'im because they are considered to be a "Shitah"; in other words, they are a "block" opinion, which suggests that the majority of Tana'im disagree with them. Instead, according to Rif, the Halachah is that one cannot take away items from the debtor, which is why the Halachah does not follow Raban Shimon ben Gamliel who maintains that one may sell the debtor's utensil after 30 days.

3) The idea that the Halachah does not follow a "Shitah" is in fact also a subject of dispute between the Rif and Rabeinu Tam in the Gemara on 69a. The Gemara there (5 lines from the top of the page) states: "Halachah was not stated but rather 'Shitah' was stated." Rashi (DH Shitah) writes that this means that several Tana'im (including Raban Shimon ben Gamliel) all maintain the same opinion. Tosfos (DH Ela) writes in the name of Rabeinu Tam that the Halachah nonetheless follows Raban Shimon ben Gamliel because of the rule that the Halachah always follows Raban Shimon ben Gamliel in the Mishnah.

The Hagahos ha'Gra (#6) on Tosfos writes that the Rif and the Rambam do not agree with the above rule. The Gra is referring to the Rif in Bava Basra, cited in my first reply, who says that the rule is "Lav Davka."

Again, we see that the Rif and Rabeinu Tam are consistent with their opinions cited by the Rosh (9:47). Rabeinu Tam maintains that the rule that the Halachah always follows Raban Shimon in the Mishnah wins out. The Rif holds that the rule that one does not rule like a "Shitah" defeats the rule that the Halachah follows Raban Shimon in the Mishnah. This is also stated by the above Tosfos (69a) in the name of Rabeinu Chananel (who was the teacher of the Rif) -- that when the Gemara says "Shitah" we do not rule like any members of the "Shitah."

Kol Tuv,

Dovid Bloom

I have found that the Vilna Gaon actually proves from Bava Metzia 102b that the Halacha does not follow Rabah bar Chanah, in the name of Rabbi Yochanan, who said in Bava Basra 174a that the Halacha always follows R. Shimon ben Gamliel in the Mishnah, with 3 exceptions.

1) This is in Biur HaGra on Shulchan Aruch Choshen Mishpat 423:3. The Gra is discussing there why the Shulchan Aruch does not pasken according to R. Shimon ben Gamliel in Bava Kama 49a. Gra cites Gemara Kesubos 77a and Bava Metzia 38b that in fact there is a dispute between the Amoraim whether R. Yochanan ever said that the Halacha follows R. Shimon ben Gamliel.

2) The above dispute involves pupils of R. Yochanan, who were earlier Amoraim, but Gra writes that all the Amoraim in Bava Metzia 102b are against R. Shimon. In addition, the fact that R. Shimon says that one splits the payment for the 13th month, proves that R. Shimon agrees with Sumchos (see (4) in my first reply). Since all over Shas the Halacha does not follow Sumchos, it follows that the Halacha does not follow R. Shimon either. So the fact that the sugya in Bava Metzia 102 does not follow R. Shimon in the final Halachic analysis, is a proof that the Halacha does not follow Rabah bar Chanah that we always pasken like R. Shimon in the Mishnah, with 3 exceptions.

3) Concerning Mishnah 10:5, we can understand why the Halacha does not follow R. Shimon, even without having resort to the above Gra. This is because Abaye said in 118b that Rabbi Yehuda, Rabbi Shimon ben Gamliel and Rabbi Shimon all maintain the same opinion. Rosh #7 writes that, if so, this is a Shittah and the Halacha does not follow any of them. This is similar to what we learnt in my previous reply, in 2).